Transit advocates say for those restrictions to work — allowing buses to move quickly enough to be a viable alternative for jilted L riders — they need to be in place all day. But the mayor rejected that view Thursday. (Todd Maisel/New York Daily News)

Mayor de Blasio shot down pleas for a 24-hour plan to avert pandemonium when the L train shuts down.

During the 15-month shutdown of the popular train between Brooklyn and Manhattan set to begin in April, the city plans to ban private cars and allow buses only on much of 14th St. — but only during rush hours.

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They also plan to require cars on the Williamsburg Bridge to carry at least three people during rush hour.

Transit advocates say for those restrictions to work — allowing buses to move quickly enough to be a viable alternative for jilted L riders — they need to be in place all day. But the mayor rejected that view Thursday.

"We think there's obviously a difference between rush hour and the other times of the day," he told reporters at an unrelated press conference in Brooklyn.

"It's a common-sense matter that the level of people traveling at rush hour, morning and evening, is different than other times of the day. We would like to minimize the disruption on 14th St. to the maximum extent possible," he said. "So this is where we think we hit the balance point."

De Blasio added that adjustments could be made later if necessary.

But advocates say if a robust plan isn't in place by the time the train shuts down, commuter chaos and gridlock will follow.

"If the city doesn't understand the emergency nature of the L train shutdown, we are going to be in for a rough experience," said John Raskin, head of the Riders Alliance.

"If the bus is not a seamless, reliable experience whenever people need to ride it, then people will abandon public transit and take Uber, which will cause more disruption than buses ever could," Raskin said. "The L train is a 24-hour-a-day train, and it's crowded all the time. We can't replicate a 24-hour-a-day transit system with measures that happen only at a certain time of day."